Heretofore, when a wellbore has been drilled in the earth and passes through a mineral-producing zone such as a reservoir which produces crude oil and/or natural gas, the mineral produced by this zone has often been produced from the wellbore to the earth's surface by setting a packer means in the wellbore above the producing zone. The packer has production tubing passing therethrough and extending from the vicinity of the producing zone to the earth's surface so that oil and/or gas flowing from the producing zone into the wellbore passes into the interior of the production tubing since the packer seals off the annulus between the outside of the production tubing and the inside of the wellbore. The produced oil and/or gas then passes through the production tubing to the earth's surface for recovery and other disposition as desired.
Sometimes, in order to increase a flow of oil and/or gas from the reservoir or producing zone into the wellbore, and thereby increase the productivity of the well itself, a process is carried out which is well known as perforating. When perforating a well a gun containing explosive charges is lowered into the wellbore adjacent the producing zone and then detonated to direct the explosion outwardly into the zone to create large channels for the oil and gas to flow into more readily. These channels also conduct the oil and gas therein quite efficiently to the wellbore itself.
If the perforating gun is lowered on a wire line through the interior of the production tubing after the packer and tubing have been set in the wellbore, the perforating gun is limited in size and power because of the inside diameter limitation of the production tubing itself. Therefore, in this "through-tubing" perforating procedure, the perforating gun or guns, because of their limited size, only shoot in one direction into the zone rather than in a plurality of directions around the 360.degree. circumference of the perforating gun.
One advantage of this procedure relates to the liquid initially present in the wellbore before the perforating operation is carried out. Sometimes this wellbore liquid is deleterious to the producing zone because it causes clays in the zone to swell or otherwise become blocked so that the productivity of oil and/or gas from the zone is actually decreased after the perforating operation. Therefore, it is desirable not to perforate through this liquid since the perforating operation would force this liquid into the zone and harm the zone itself. In the through-tubing procedure, the wellbore liquid below the packer can be removed and replaced with a nondeleterious liquid while still keeping heavier weighted liquid above the packer which could be deleterious to the zone but which is beneficial above the packer because it lessens the chance of a well blow-out should the packer itself fail to seal adequately between the outside of the production tubing and the inside of the wellbore. Thus, through-tubing perforating is a safe procedure from a well control point of view but leaves something to be desired when a large amount of perforating in various directions around the wellbore is desired.
Another perforating procedure often used is "tubing conveyed" perforating wherein the perforating gun or guns are first fixed to and below the packer before it is placed in the wellbore so that the perforating guns are not limited by the internal diameter of the production tubing but rather only by the internal diameter of the much larger wellbore itself. This way, much larger perforating guns can be employed and a plurality of charges spaced around the periphery of the perforating gun or guns used so that perforations can be created around the 360.degree. circumference of the wellbore as desired. For example, the charges can be spaced 90.degree. apart, 120.degree. apart, or any other spaced angle around the periphery of the gun so that channels are created at a plurality of places around the interior of the wellbore. This is a substantial advantage for this procedure. However, if it is desirable in this procedure to remove the initial wellbore liquid because it is deleterious to the producing zone or zones to be perforated, then this liquid must be removed from the entire wellbore before the packer-perforating gun assembly is lowered into the wellbore and set into place. Thus, the same type of liquid is present above and below the packer and thus runs a greater risk of a well kick or blow-out should, for example, the packer leak and high pressure natural gas escape around the packer into an upper section of the wellbore. This is so because the upper section of the wellbore will not contain a weighted liquid which could prevent the kick or blow-out but rather only contains the same liquid desired below the packer between the perforating gun and zone or zones to be perforated. Therefore, the tubing conveyed perforating procedure leaves something to be desired as well.
However, the tubing conveyed procedure has the distinct advantage that the larger perforating guns can penetrate further out into the formation and leave larger channels therein. This is known as increased perforation efficiency which, in turn, leads to increased production of desirable fluids from the perforated formation.